In high-speed ICs, it is often necessary to drive signals, i.e. translate signal voltages, from one voltage domain to another (higher or lower) voltage domain. For example, in high-speed ICs, it may be required to translate signal voltages between an “I/O” voltage domain and a “core” voltage domain, or between a “clean” analog voltage domain and a “dirty” digital voltage domain. The signals to be translated may be single-ended signals or differential signals. In the case of single-ended signals, the voltage translation effectively changes the reference voltage of the signals of interest, and in the case of differential signals, the voltage translation alters the common mode voltage of the signals.
Various implementations of a voltage translation circuit exist in the prior art; the following is a discussion of three conventional implementations. The first implementation translates signal voltage from an input voltage domain to an output voltage domain by using large series AC-coupling capacitors tied to large resistors. In effect, the capacitors block the DC components and pass the AC components of the signal, and generate the output signal in the output voltage domain. The resistors and capacitors in this implementation are required to be large to pass the low-frequency content without undesirable “droop” in the frequency response. However, the use of large resistors or large capacitors results in large circuit area and high cost, which are undesirable. In addition, the parasitic capacitance of the AC-coupling capacitor tends to negatively effect the high frequency performance of the circuit.
The second implementation uses emitter followers or source followers to shift voltages between input and output domains, hence eliminating the need for large passive components, like in the first implementation, with minimal compromise of speed. However, this implementation does not readily allow an arbitrary shift in the voltage level of a signal, and particularly, with regard to differential signals, this implementation has an undesirable property that the output common mode voltage is proportional to the input common mode voltage.
The third implementation employs a transconductance amplifier to convert the input voltage signal to a current signal that is delivered across the voltage domain boundary. At the output side of this implementation, either a resistive load or another transconductance stage may be used to convert the current signal back to a voltage signal. This approach also does not require large passive components, but is often much slower than the two above-discussed implementations because it often requires slower high-voltage devices or p-type devices in the signal path.